Azeotrope-like compositions of difluoromethane

ABSTRACT

The invention relates to azeotropic and azeotrope-like mixtures of HFC-32, HCl, and Cl 2 , and processes for separating the azeotrope-like mixtures of HFC-32 and HCL and HFC-32, HCL and Cl 2 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims priority to, U.S.application Ser. No. 10/911,012, filed Aug. 4, 2004, which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to novel compositions comprisingdifluoromethane.

BACKGROUND OF THE INVENTION

Many techniques are known for the production of hydrofluorocarbons(HFCs), and in particular difluoromethane (HFC-32). Certain priormethods involve fluorinating one or more chlorinated organic compoundsto produce difluoromethane. The reaction product stream typically alsocontains byproducts and one or more unreacted starting materials. Thedesired difluoromethane is then recovered from the reaction productstream through one or more separation processes, such as distillation.For example, in certain difluoromethane production processes, achlorinated organic compound, such as, for example, dichloromethane(HCC-30), and a fluorinating agent, such as, for example, hydrogenfluoride (HF), are reacted, usually after preheating, in the presence ofa fluorination catalyst to generate a reaction product stream.

Distillation is well known in the art for separating the components ofsuch reaction product streams and typically involves the use ofdistillation means, such as a packed column or one with trays, operatedat pressures and temperatures selected to separate the reaction productstream into a stream relatively rich in the desired compound and streamrelatively rich in compounds that are not desired in the finishedproduct, such as unreacted components and unwanted byproducts. However,standard distillation techniques are generally ineffective to separatethe components of azeotropic mixtures. Azeotropic binary compositionsconsisting of HFC-32 and chlorine are disclosed in U.S. Pat. No.6,099,694, which is assigned to the assignee of the present inventionand which is incorporated herein by reference.

The desirability of a fluorination process is generally linked to theyield and product purity resulting from the process. For example, if thedesired product is the difluoromethane, the amount of such product whichis recovered from the reaction product should ordinarily be as high aspossible, and the type and amount of impurities contained in the finalproduct stream should be as low as possible. While prior processes haveachieved a certain level of success as measured by yield and productpurity, applicants have discovered that that certain features of theprior art may raise barriers against continuing improvement of productyield and purity.

SUMMARY OF THE INVENTION

Applicants have discovered that difluoromethane and hydrogen chlorideform a binary azeotrope-like composition and that difluoromethane,hydrogen chloride and chlorine form a ternary azeotrope-likecomposition. This discovery is particularly important because in theproduction of difluoromethane from chlorinated reactants, andparticularly when chlorine is used as a catalyst regenerator and/oractivator, difluoromethane, hydrogen chloride and chlorine are presentin the reaction product. In addition to the discovery of theazeotrope-like compositions, therefore, applicants have discoveredimproved processes for the production of difluoromethane, includingmethods to eliminate or reduce the levels of hydrogen chloride and/orchlorine from the reaction product stream, and/or to recover anincreased amount of hydrogen chloride and/or chlorine from the reactionproduct stream.

In preferred embodiments, the processes comprise reacting a chlorinatedcompound, preferably dichloromethane (HCC-30), with a fluorinatingagent, preferably HF, to produce a product mixture comprising HFC-32 andhydrogen chloride. In certain embodiments, especially when chlorine isused as an activating agent, chlorine is also present in the reactionproduct stream. In one preferred embodiment of the method aspects of thepresent invention, one or more of the present azeotrope andazeotrope-like compositions are separated from the reaction mixture, andoptionally but preferably the component parts thereof are thereafterseparated to produce compositions enriched in HFC-32 and/or enriched inHCl, and/or enriched in a mixture of HCl and Cl₂, As used herein, thereference to enriched refers to the component having a higherconcentration in the enriched composition relative to the concentrationof that component in the azeotrope or azeotrope-like composition.

The azeotrope-like compositions are useful also in processes for theremoval of impurities from HFC-32.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The Compositions

The present compositions are azeotrope-like compositions. As usedherein, the term “azeotrope-like” is intended in its broad sense toinclude both compositions that are strictly azeotropic and compositionsthat behave like azeotropic mixtures. From fundamental principles, thethermodynamic state of a fluid is defined by pressure, temperature,liquid composition, and vapor composition. An azeotropic mixture is asystem of two or more components in which the liquid composition andvapor composition are equal at the state pressure and temperature. Inpractice, this means that the components of an azeotropic mixture areconstant boiling and cannot be separated during distillation.

As the term is used herein, “azeotrope-like” compositions behave likeazeotropic mixtures, that is, they are constant boiling or essentiallyconstant boiling. In other words, for azeotrope-like compositions, thecomposition of the vapor formed during boiling or evaporation isidentical, or substantially identical, to the original liquidcomposition. Thus, with boiling or evaporation, the liquid compositionchanges, if at all, only to a minimal or negligible extent. This is tobe contrasted with non-azeotrope-like compositions in which, duringboiling or evaporation, the liquid composition changes to a substantialdegree. All azeotrope-like compositions of the invention within theindicated ranges as well as certain compositions outside these rangesare azeotrope-like.

The azeotrope-like compositions of the invention may include additionalcomponents that do not form new azeotropic or azeotrope-like systems, oradditional components that are not in the first distillation cut. Thefirst distillation cut is the first cut taken after the distillationcolumn displays steady state operation under total reflux conditions.One way to determine whether the addition of a component forms a newazeotropic or azeotrope-like system so as to be outside of thisinvention is to distill a sample of the composition with the componentunder conditions that would be expected to separate a non-azeotropicmixture into its separate components. If the mixture containing theadditional component is non-azeotropic or non-azeotrope-like, theadditional component will fractionate from the azeotropic orazeotrope-like components. If the mixture is azeotrope-like, some finiteamount of a first distillation cut will be obtained that contains all ofthe mixture components that is constant boiling or behaves as a singlesubstance.

It follows from this that another characteristic of azeotrope-likecompositions is that there is a range of compositions containing thesame components in varying proportions that are azeotrope-like orconstant boiling. All such compositions are intended to be covered bythe terms “azeotrope-like” and “constant boiling”. As an example, it iswell known that at differing pressures, the composition of a givenazeotrope will vary at least slightly, as does the boiling point of thecomposition. Thus, an azeotrope of A and B represents a unique type ofrelationship, but with a variable composition depending on temperatureand/or pressure. It follows that, for azeotrope-like compositions, thereis a range of compositions containing the same components in varyingproportions that are azeotrope-like. All such compositions are intendedto be covered by the term azeotrope-like as used herein.

Difluoromethane/HCl/Cl₂

One embodiment of the invention provides azeotrope-like compositionscomprising difluoromethane, hydrogen chloride and chlorine. Preferably,the novel azeotrope-like compositions of this embodiment compriseeffective amounts of the HFC-32, hydrogen chloride and chlorine. Theterm “effective amounts” as used herein means the amount of eachcomponent which upon combination with the other component or components,results in the formation of the present azeotrope-like compositions.

These embodiments preferably provide azeotrope-like compositionscomprising, and preferably consisting essentially of, from about 1 toabout 20 parts by weight HFC-32, from about 70 to about 99.9 parts byweight of hydrogen chloride, and from about 1 to about 13 parts byweight chlorine. Such compositions are preferably characterized by asubstantially constant vapor pressures of about 45±3 psia, morepreferably about 45±2 psia, and even more preferably about 45 psia at atemperature of about 59° C.

The preferred, more preferred, and most preferred compositions of thisembodiment are set forth in Table 1. The numerical ranges in Table 1 areto be understood to be prefaced by the term “about”. TABLE 1 More MostPreferred Preferred Preferred Components (pbw) (pbw) (pbw) HFC-320.25-20 0.25-15 0.25-10 HCl    70-99.5    80-99.5    95-99.5 Cl₂ 0.25-130.25-5  0.25-3 

Difluoromethane and Hydrogen Chloride

One embodiment of the invention provides azeotrope-like compositionscomprising, and preferably consisting essentially of, difluoromethaneand hydrogen chloride. Preferably, the novel azeotrope-like compositionsof this embodiment comprise effective amounts of difluoromethane andhydrogen chloride. Such compositions preferably comprise, and even morepreferably consist essentially of, from about 1 to about 20 parts byweight HFC-32, and from about 80 to about 99 parts by weight of HCl. Inmany embodiments, such preferred compositions are characterized by asubstantially constant vapor pressure of about 50±4 psia, morepreferably about 50±2 psia, and even more preferably about 50 psia at atemperature of about −59.3° C.

The preferred, more preferred, and most preferred compositions of thisembodiment are set forth in Table 2A. The numerical ranges in Table 2Aare to be understood to be prefaced by the term “about”. TABLE 2A MoreMost Preferred Preferred Preferred Components (pbw) (pbw) (pbw) HFC-321-20 1-10 1-5 HCl 80-99  90-99  95-99

The following Table 2B provides vapor pressure data for the HFC-32/HClcompositions according to preferred embodiments of the presentinvention. TABLE 2B HFC-32/HCl Azeotrope HFC-32/HCl Wt Ratios VaporPressure (psia) at −59.2° C. 100/0 53.6 97.7/2.3 53.1 88.9/11/1 50.684.0/16 48.9 80.3/19.7 48.0The Methods

Fluorination Processes

In one embodiment, the method aspects of the present invention includeimproved fluorination processes comprising the steps of (a) reacting oneor more reactants to produce a reaction product comprising at leastHFC-32 and HCl, removing from said reaction product an azeotrope-likecomposition comprising HFC-32 and HCl. Optionally, but preferably, themethods also include separating at least a portion of the HCl from saidremoved azeotrope or azeotrope-like HFC-32/HCl composition to produce acomposition enriched in HFC-32. Optionally, but preferably, the methodsmay also include producing from said azeotrope or azeotrope-likecomposition a composition enriched in HCl.

In another embodiment, the method aspects of the present inventioninclude improved fluorination processes comprising the steps of (a)reacting one or more reactants to produce a reaction product comprisingat least HFC-32, HCl and Cl₂, removing from said reaction product anazeotrope or azeotrope-like composition comprising HFC-32, HCl and Cl₂.Optionally, but preferably, the methods also include separating at leasta portion of the HCl and/or the Cl₂ from said removed azeotrope orazeotrope-like HFC-32/HCl/Cl₂ composition to produce a compositionenriched in HFC-32. The term “enriched” is used herein to refer to thecondition during the distillation of a mixture in which theconcentration of one component in either the distillate or a bottomsproduct is higher relative to its concentration in the mixture.

Optionally, but preferably, the methods may also include producing fromsaid azeotrope or azeotrope-like composition a composition enriched inCl₂. When one or more of the optional separations step is used, it isgenerally preferred that at least a portion of the Cl₂ so separated isrecycled to the chlorination reaction.

The fluorination step of the present invention can be carried out inaccordance with any process known in the art, and particulars of allsuch processes are within the scope of the present invention and neednot be explained in detail here. In is sufficient to note that thepresent processes generally a mixture of halogenated compounds,including HFC-32, HCl and other byproducts, including in certainembodiments Cl₂. Thus, the mixture of reactants, byproducts and reactionintermediates of the process may be present along with the HCl, Cl₂ andHFC-32 in the mixture.

Accordingly, in one embodiment, the present invention provides a processfor separating HFC-32 from an HFC-32/HCl₂ azeotrope-like mixture. Inother embodiments, the present invention provides a process forseparating HFC-32 from an HFC-32/HCl/Cl₂ azeotropic or azeotrope-likemixture. It will be appreciated by those skilled in the art that severaltechniques are know and available for separating azeotropic orazeotrope-like compositions into compositions enriched in one or more ofthe components thereof. For example, liquid-liquid phase separationtechniques are generally effective in this regard and are believed toadaptable for use in accordance with the present invention. In otherembodiments, the present process comprises, consists essentially of, orconsists of the steps of:

(A) distilling a mixture comprising HFC-32 and HCl, or HFC-32 and amixture of HCl and Cl₂, at a first pressure to produce a streamconsisting of an azeotrope-like composition of the present invention;and

(B) distilling said azeotrope-like composition of the present inventionat a second pressure to produce a stream enriched in any one of HFC-32,HCl and Cl₂. The distillation steps of the present methods may beperformed using a single distillation column or a series of distillationcolumns. In embodiments wherein a single distillation column is used,the methods of the present invention are typically performed as batchdistillations. The mixture may be fed, for example, into a batchdistillation column operating at a first pressure. The azeotrope likecomposition of the present invention is then collected and refed intothe column at a second pressure. Preferably, the methods of the presentinvention are performed using a series of distillation columns, meaningat least two columns, operating at different pressures in a batch orcontinuous distillation. Examples of distillation columns and methodssuitable for use in the present invention are disclosed in U.S. Pat. No.5,918,481 (issued to AlliedSignal), which is incorporated herein byreference.

Whether the distillation process is continuous or batch, the pressuresat which the distillations are conducted preferably are such thatconventional distillation apparatus can be used. The higher distillationpressure generally may range from about 40 to about 400 psia, preferablyfrom about 60 to about 200 psia. The lower distillation pressuregenerally may range from about 10 psia to about 40 psia, preferably fromabout 15 psia to less than about 30 psia.

The temperatures at which these distillations are performed are directlyrelated to the boiling points and pressures used, and are well withinthe scope of knowledge of one skilled in the art in view of theteachings contained herein.

In certain other embodiments, the present invention provides a methodfor removing HFC-32 from a mixture containing HFC-32 and at least oneimpurity. As used herein, the term “impurity” refers to any compoundpresent in a mixture with HFC-32 from which it is desirable, for a givenapplication, to separate the HFC-32. Preferably, the impurity itselfdoes not form an azeotrope-like mixture with HFC-32, HCl, Cl₂ or amixture of HCl and Cl₂. Typical impurities include other halocarbonswhich may be miscible with HFC-32 such as, for example,chlorofluoromethane (HCFC-31) or dichloromethane (HCC-30).

The method for separating HFC-32 and at least an impurity preferablycomprises adding HCl or a mixture of HCl and Cl₂ to the mixture in anamount sufficient to form an azeotrope-like composition of the HFC-32and the HCl or a mixture of HCl and Cl₂, and then separating theazeotrope-like composition from the mixture.

The azeotropic composition of the present invention may be separatedfrom the mixture comprising the impurity by any of a number ofconventional methods. Examples of separation methods include, forexample, distillation, scrubbing, other art-recognized separating means,and combinations of two or more thereof. Any mixture containing HFC-32and at least one impurity may be used in the present method. While suchmixtures may be provided via any conventional source, in certainpreferred embodiments, the mixtures are reaction products resulting froma manufacturing process, most notably, the production of HFC-32.

Those of skill in the art will recognize that the amount of HCl or amixture of HCl and Cl₂ to be added to the mixture, and to form anazeotrope-like composition, will depend on the conditions under whichthe azeotrope-like composition is formed. In light of the disclosureherein, those of skill in the art will be readily able to determine theamounts of HCl or a mixture of HCl and Cl₂ necessary to formazeotrope-like compositions with HFC-32 under a wide range of pressuresand temperatures.

EXAMPLES

The following non-limiting examples serve to illustrate the invention.

Example 1

Binary compositions consisting essentially of HFC-32 and HCl are blendedto form homogeneous mixtures having different compositions. The vaporpressures of the mixtures were measured at −59.2° C.

Table 2b above shows the vapor pressure measurement of HFC-32/HCl as afunction of composition at a substantially constant temperature of−52.9° C. From this data it is observed that at −59.2° C. thecomposition exhibits azeotrope-like properties from about 1 to about 20weight percent HFC-32. Based on further observations made during theexperiment, it is determined that the composition at which the vaporpressure is the maximum is from greater than 0 up to about 2.3 weightpercent HFC-32 at −59.2° C.

Example 2

Binary compositions consisting essentially of HFC-32, Cl₂ and HCl areblended to form homogeneous mixtures having different compositions. Thevapor pressures of the mixtures were measured at −59.2° C.

Table 3 shows the vapor pressure measurement of HFC-32/HCl/Cl₂ as afunction of composition at a constant temperature of −59.2° C. From thisdata it is observed that at −59.2° C. the composition exhibitsazeotrope-like properties at compositions comprising from about 17 toabout 20 weight percent HFC-32 and from about 1 to about 13 weightpercent Cl₂, the balance being HCl. TABLE 3 WEIGHT PERCENT (remainderHFC-32) HFC-32 CL₂ VAPOR PRESSURE (PSIA) AT −59.2° C. 19.4 1.5 46.3 18.36.9 45.2 17.2 12.3 43.8

1.-14. (canceled)
 15. An azeotrope-like composition, which consistsessentially of difluoromethane (HFC-32), HCl and Cl₂.
 16. Thecomposition of claim 15 having a vapor pressure of about 45±3 psia atabout −59° C.
 17. The composition of claim 16 consisting essentially offrom about 0.25 to about 20 weight percent HFC-32, from about 70 toabout 99.5 weight percent HCl and from about 0.25 to about 13 weightpercent Cl₂.
 18. The composition of claim 16 which consists essentiallyof from about 0.25 to about 15 weight percent HFC-32, from about 80 toabout 99.5 weight percent HCl and from about 0.25 to about 5 weightpercent Cl₂.
 19. The composition of claim 16 which consists essentiallyof from about 0.25 to about 10 weight percent HFC-32, from about 95 toabout 99.5 weight percent HCl and from about 0.25 to about 3 weightpercent Cl₂. 20.-31. (canceled)
 32. A method of forming an azeotropic orazeotrope-like composition comprising combining HFC-32, HCl and Cl₂ inamounts sufficient and under conditions effective to form azeotrope-likecomposition containing HFC-32, HCl and Cl₂.
 33. A method of forming anazeotropic or azeotrope-like composition which consists essentially ofcombining from about 1 to about 20 weight percent HFC-32, from about 70to about 98 weight percent HCl and from about 1 to about 13 weightpercent Cl₂, which composition has a vapor pressure of from about 45psia to about 55 psia at a temperature of about 59° C.